Color rules need to be at least 1 point minimum when printing in two or more colors.
1/C rules need to be a 1/4 point minimum.
Whenever very thin or small objects such as rules are in two or more colors, variation becomes more pronounced. To reduce the chances of undesirable registration problems use rules to one point or larger.
Any rule that is 4/C can result in poorer than anticipated product quality, and increased paper waste at press which could affect your schedule attainment and/or cost of your product.
Thin rules may not reproduce satisfactorily, especially when screened or when printing on a screened background.
Reversed Rules
Reversed rules should be a minimum of 1 point.
To limit problems with reversed out elements, build background fill colors with as few separations as possible. When reversing rules out of a background tint, set the background tint to 30% or more.